Hi ptwlee888,
> how can i print out the levels from the bottom up?
> can i use queue to print it out?
A dequeue will be more convenient ...
Traverse using BFS ... Insert all elements at the tail of dequeue ... Print from the tail of the dequeue

> what if i used stack structure instead of queue?
> what is the order then?
> what kind of traversal will this be after i used stack?
BFS push on stack ... same result

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